Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
  • H04N1/40—Picture signal circuits
  • H04N1/405—Halftoning, i.e. converting the picture signal of a continuous-tone original into a corresponding signal showing only two levels
  • H04N1/4055—Halftoning, i.e. converting the picture signal of a continuous-tone original into a corresponding signal showing only two levels producing a clustered dots or a size modulated halftone pattern
  • H04N1/4057—Halftoning, i.e. converting the picture signal of a continuous-tone original into a corresponding signal showing only two levels producing a clustered dots or a size modulated halftone pattern the pattern being a mixture of differently sized sub-patterns, e.g. spots having only a few different diameters

Definitions

• the invention relates to a method for printing a halftone image in accordance with image data, each image datum describing the halftone level of a raster point of a template image, in which processing frames are used, each processing frame having positions for the image data of a plurality of adjacent raster points to which the image data are transferred to the processing frames in the arrangement of the halftone dots described by them, in which color level values are obtained from the image data contained in the processing frames, the printing positions of a printing raster are assigned, and in which color dots are optionally used to generate the halftone levels are printed with graded color gradations at the print positions, the color gradation of each color point being set in accordance with the color step value assigned to the respective print position.
• printing is basically to be understood to mean the application of color dots to a recording medium.
• each image date describing the halftone level of a halftone dot of a template image.
• a threshold value assignment is used to assign a matrix to each image data item, which matrix comprises a number of print positions of a print screen.
• the print positions of each matrix can be assigned with color dots of graded color gradations (different sizes).
• the matrix In order to be able to produce a sufficiently large number of halftone levels in this known method, the matrix must comprise a correspondingly high number of printing positions. As the number of print positions per matrix increases, however, the spatial resolution of the reproduced halftone image decreases considerably in comparison to the original image. In order to still achieve good image reproduction, Therefore, in the known method, printheads with a very high resolution have to be used, which make the price of a corresponding printing device more expensive.
• the invention is based on a method for printing a halftone image known from EP 0 240 202 A1, in which there is also a template image in the form of image data, each image date being assigned to a raster point of the template image and describing its halftone level.
• processing frames are used, each processing frame having positions for recording a plurality of image data from adjacent raster points of the original image; the image data are taken over into the positions of the processing frames in the arrangement of the raster points described by them.
• Color level values are obtained from the image data contained in the processing frames by averaging and these printing positions are assigned to a printing screen.
• a color point with graded color gradations can optionally be output to generate the halftone levels; the color gradation of each color point is determined by a color step value assigned to the respective printing position.
• the image data of a processing frame are evaluated together by averaging or interpolation, and in this way a color level value is obtained which is assigned to a number of printing positions in the printing raster corresponding to the number of image data per processing frame.
• the invention has for its object to provide a method for printing a halftone image with which a high number of different halftone levels can be generated and a high spatial resolution is given in each area.
• a reference pattern with a predetermined arrangement of positions with predetermined color level values is assigned to each halftone step, the structure and position numbers of the reference patterns corresponding to the processing frames,
• a color level value for a print position is obtained from each image date by assigning the color level value present to the corresponding print position in the processing frame in accordance with the halftone level described by the image date and the position number of its position in the processing frame .
• Reference patterns with a relatively large number of positions containing color level values can be used, so that a high number of finely graduated halftone levels can be generated.
• the method according to the invention advantageously enables a resolution in the area of contrast jumps in the original image which only determined by the fineness of the print screen and depends on the number of different color level values. A blurred representation in the area of contrast jumps is thus advantageously avoided without the printing device requiring an increased resolution in comparison to the rasterization of the original image.
• An equalization of the optical impression of the halftone levels that can be produced is achieved according to an advantageous development of the method according to the invention in that at most two different color level values are contained in each reference pattern.
• a further advantageous development of the method according to the invention is that reference patterns are used which differ from halftone level to halftone level in the assignment of the positions in that each position is assigned a color level value that is one level higher than the reference pattern of the next lower halftone level, the sequence of the assignment with a color level value increased by level in each case being determined by the sequence of the position numbers with increasing intensity of the halftone levels.
• Contrast jumps and regular color impressions (textures) for the human eye are thus advantageously avoided.
• the uniformity of an area printed with the same or approximately the same halftone levels is further improved in that the position numbers are defined in a processing frame in such a way that the sum of the distances between the positions with successive position numbers is maximum, whereby also the positions immediately neighboring processing framework can be considered.
• processing frames and reference patterns with a symmetrical diamond-shaped structure are used.
• processing frames and reference patterns each with thirteen positions, have proven to be particularly advantageous.
• FIG. 1 shows a schematic illustration of the method according to the invention
• FIG. 2 shows a schematic illustration for determining position numbers
• FIG. 3 position numbers in processing frames or reference patterns
• FIG. 4 shows a preferred processing frame or a preferred reference pattern
• FIG. 5 shows color points assigned to color level values
• FIG. 6 reference patterns assigned to different halftone levels
• Figure 9 shows the conditions in a contrast jump.
• FIG. 1 there is a template image VB in the form of digital image data BD.
• the image data BD can have been generated, for example, by optically scanning the original image VB.
• Each individual image data of the image data BD represents a halftone dot RP with a halftone level located at a specific point on the template image VB.
• a halftone level is a color gradation of the color of the original image (in the black and white image, for example, different shades of gray) at this grid point.
• image data BD from a plurality of adjacent raster points RP are each assigned processing frames in the arrangement in which the raster points RP they represent are arranged on the template image VB. As explained in more detail later in connection with FIGS.
• a color level value is obtained from each image data in an allocation device ZE by accessing reference patterns stored in a memory SP.
• the color level value determined in this way is written into a print data memory DS in a memory location which is assigned to the raster point represented by the respective image data.
• the data contained in the print data memory DS are fed to a print head DK which contains a plurality of individually controllable printing elements.
• This can be, for example, a thermal print head in which the control energy of the individual printing elements in each case causes a color point of different size and thus different color gradation on a recording medium AT.
• FIG. 2 schematically shows the procedure for determining position numbers in a processing frame VR1, further processing frames VR2 to VR5 which are directly adjacent to the processing frame VR1 also being considered.
• the positions of the processing frames VR1 to VR5 are provided with item numbers in any combinable order. For example, position numbers 01 and 02 have already been entered in FIG. 2, the following position numbers 03 to 13 being set arbitrarily until the processing frames are completely filled with position numbers. The first position number 01 is set arbitrarily and then the following position numbers are set accordingly. The distances in each case determined and added to the position numbers already set. The order of item numbers is considered the most favorable, the sum of the distances determined in this way being maximum.
• FIG. 3 shows fifteen sequences of position numbers determined in the manner described above, which are equivalent in terms of the distances explained above. Printing tests have shown that the processing frame V15 with its sequence of position numbers is the cheapest for the method according to the invention, because with an increasing distribution of the same color dots according to the enumerated position numbers, a particularly even impression is created for the human eye with halftone areas .
• FIG. 4 shows a reference pattern R which corresponds in its structure and the arrangement of positions containing color level values (position numbers x to 13) to the processing frame V15 according to FIG. 3.
• Both the processing frame V15 and the reference pattern R have a diamond-shaped, symmetrical structure and have a total of thirteen positions.
• a color level value FSO means that no ink is applied at the respective printing position (the recording medium remains unprinted there).
• color dots FP1 to FP5 are generated with increasing size.
• the Arabic numeral ⁇ es color is added gradually in the value in the figure 6 Darges t ellten reference patterns only one.
• the number "5" symbolizes z. B. the color level value FS5.
• the reference patterns R1 to R13 differ in that, according to the item numbers (FIG. 4) 01 to 13, a further color point FP1 is set with each halftone level HSO to HS65 that increases in intensity.
• a is set in accordance with item numbers 01 to 13
• Color level value increased from FSl to FS2.
• the other halftone levels HS27 to HS65 are formed in an analogous manner.
• the different color level values FS1 to FS5 can also be implemented by increasing optical densities of the color points FPl to FP5.
• the different sizes of the color dots FP1 to FP5 are realized by appropriately controlled energy supplies to the individual heating elements of the thermal print head.
• the template image VB shown symbolically consists of a plurality of raster points RP, a section AS of the template image VB being shown enlarged (AS1).
• the halftone level of each raster point RP is represented by an image data BU.
• image data BU For reasons of space, only the Arabic number of the respective halftone level is shown; for example, "18" means the halftone level HS18.
• Section AS1 shows the combination of thirteen image data in each case in a processing frame V15 (cf. FIG. 3).
• the processing frames border one another seamlessly, so that virtually the entire template image area is covered by processing frames or all of them Image data in the processing frame are adopted.
• the assignment made in the assignment device ZE (FIG. 1) is explained below with reference to three example image data selected in the processing frame V15 and marked with "0". The top one, on position number 07 of the
• Processing frame V15 arranged image data BU represents a halftone level HS20.
• the color level value 2 (FS2) is entered at position number 07 in the reference pattern R20 assigned to the halftone level HS20.
• the color level value FS 2 is therefore assigned to this image data and written into the print data memory DS in a corresponding position, as is indicated in a print data memory section AS2.
• One image data BU representing the halftone level HS18 is arranged on the item numbers 08 and 12 of the processing frame V15; In accordance with the reference pattern R18 assigned to the halftone level HS18, these positions in the print data memory DS each receive the color level value 1 (FSl). In this way, all print data is processed, so that the print data storage section AS2 finally has the assignment shown in FIG. 7 with color level values.
• the print head DK is loaded with the color step values obtained in this way.
• FIG. 8 shows a print result obtained in this way, only a section AS3 of the reproduction image WB being shown here as well.
• a print position DPO remained without ink application because the corresponding memory location of the memory (AS2 in FIG. 7) contained the color level value FSO.
• the viewer receives a relatively uniform image from the detail AS3, which is neither patterned by horizontal nor vertical color point concentrations (textures) and contains no significant jumps in contrast. This impression corresponds to the image data according to section AS1 (FIG. 7), which represent a relatively narrow range of halftone levels.
• FIG. 9 shows the conditions in the event of a contrast jump occurring within the original image VB.
• This jump in contrast has the course of the thin line KS in an analog image.
• the contrast jump runs along the line KSD.
• An area with a relatively intense gray value (halftone level 48) is shown on the left of the line KSD and an area with a relatively low intensity (halftone level 17) is shown on the right of the line KSD.
• each individual image date BU is evaluated and a color level value is assigned to each image date, so that the contrast jump along the line KDS to an arrangement of color level values according to reference pattern R17 for the halftone level HS17 to the right of the line KSD and to an arrangement of color level values according to reference pattern R48 for the halftone level 48 to the left of the line KSD.
• reference pattern R17 for the halftone level HS17 to the right of the line KSD
• reference pattern R48 for the halftone level 48 to the left of the line KSD.
• FIG. 9 shows that in the area of homogeneous color gradations - e.g. B. to the left of the line KSD at the halftone step 48 - the arrangement of the color step values is identical to the arrangement of the color step values in the reference pattern R48 or a corresponding output of color points FP3 and FP4. The same applies to the area to the right of the KSD line.
• the areas of homogeneous halftone levels are therefore more homogeneous Arranged arrangement of color points that differ by a maximum of one color level value. Since the arrangement of the color level values in the reference patterns has no concentrations of color level values of higher or lower values in the horizontal or vertical image direction, the viewer has a uniform gray impression without disturbing textures in the reproduction image WB.

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• Engineering & Computer Science (AREA)
• Multimedia (AREA)
• Signal Processing (AREA)
• Facsimile Image Signal Circuits (AREA)
• Image Processing (AREA)

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Citations (7)

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• 1990
  • 1990-08-13 DE DE4025793A patent/DE4025793A1/de active Granted
• 1991
  • 1991-07-26 US US07/969,200 patent/US5365348A/en not_active Expired - Lifetime
  • 1991-07-26 DE DE59107132T patent/DE59107132D1/de not_active Expired - Fee Related
  • 1991-07-26 EP EP91913003A patent/EP0543833B1/de not_active Expired - Lifetime
  • 1991-07-26 WO PCT/DE1991/000612 patent/WO1992003885A1/de active IP Right Grant

Patent Citations (7)

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